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THE BLAST FURNACE.                                        73
number of cubic feet of air per minute and the same cubic feet of air per ton of coke, while 'the volume of tunnel head gas will likewise be the same as before per minute and per ton of coke. If the gas were of equal quality in both cases, the amount needed for stoves and engines and the amount available for surplus power would not be greatly changed by a reduction in the coke consumption.
The discussion of the matter is taken up in the following order:
(1)   Calculation on the volume and heat value of the gas.
(2)   Rough methods of corroborating these calculations.
(3)   Amount of steam in gas.
(4)   Energy needed to heat the blast.
(5)   Results of burning gas under boilers.
(6)   Production of power in steam engines.   '
(7)   Production of power in gas engines.
SEC. Iln.—Volume and value of the tunnel head gas.—Table 1I-E gives the method of calculating the composition and volume of tunnel head gas under certain assumed conditions, while Table II-F arbitrarily assumes several different sets of furnace conditions, so as to constitute a series for comparing the effect of different factors: thus two columns are alike in amount of fuel, stone, and atmospheric moisture, but different in carbon ratio; another two have the same fuel, stone and carbon ratio, but differ in moisture. The effect of an increase in the amount of limestone is difficult to calculate. In E and F two extreme suppositions have been made: in E it is assumed that all the carbonic acid in the additional weight of stone is driven off unchanged; in F it is assumed that this gas reacts upon the coke and is all converted into carbonic oxide. Neither of these extremes is true, but a portion of the carbonic acid would pass off unaltered and a portion would react upon the carbon. The column with 1700 pounds of coke assumes conditions similar to those given by Gayley in his experiments on refrigera.-tion; while the two columns showing 3300 pounds of fuel per ton of iron illustrate practice at furnaces where the ore carries 20 per cent, of silica, 1.5 per cent, of sulphur after roasting, and only 42 per cent, of iron. Viewing each set of conditions as a separate problem, the volume and calorific value of the tunnel head gases have been worked out. It is assumed that the gas contains 1.5 per